This invention relates to a pipe joint used to connect plastic pipes with each other, and particularly to an electrofusion joint wherein a coiled electric heating wire (referred to as `coil` hereinafter) is embedded therein and which fuses and adheres at the contact surface to the pipes by thermal fusion by application of an electric current to the coil.
The electrofusion joint has recently been developed as a pipe joint for plastic pipes which are used in water supply and drainage pipes and a gas pipe. It comprises a molded product wherein a coil is embedded in a cylindrical body. The pipes are inserted into the joint from the edges at both sides thereof, and an electric current is applied to the joint to cause thermal fusion at the contact surface with the pipes to accomplish fusion/adhesion between the joint and the pipes. Connection of the pipes may simply be carried out by use of the joint. The operation comprises only application of an electric current to the joint after insertion of the pipes into the joint from the edges at both sides thereof. Thus, the joint has such advantages that the connection work may simply and rapidly be carried out, work efficiency is improved, and connection may be carried out with relatively high reliability as compared to a conventional pipe joint wherein connection of the pipes is carried out by fitting the joint to the pipes which have previously been fused by heating at the contact surface. However, there is a problem in the degree of adherence of the coil to the resin forming the cylindrical body. The thermal conductivity between the coil and the resin depends on the degree of adherence. If the degree of adherence is ununiform, the resin is fused in some parts and is not fused in other parts. Thus, there appears a dispersion in fusion/adhesion properties. The degree of adherence is affected by the temperature of the joint (The degree of adherence decreases by the difference in the thermal expansion of the resin and the coil if the temperature rises, whereas, it increases by the difference in shrinkage when the temperature lowers), the pressure of the fused resin, the residual stress at molding, deterioration of the resin with time, etc. Also, the resin is sometimes peeled off from the coil by shrinkage of the resin in the process of cooling and solidification at molding. If a joint having inferior adherence between the coil and the resin is used, the coil is abnormally heated because the heat is not well absorbed by the coil. This causes degradation and finally wax formation of the peripheral resin. These cause a decrease in fusion/adhesion properties.